1. Field of the Invention
This invention relates generally to cathodic protection systems for protecting metal objects, which are buried in soil, from corrosion. The invention more particularly relates to a circuit for monitoring electrical parameters associated with the buried object and a coupon and reference electrode which are conventionally installed to provide information about the effectiveness of the cathodic protection.
2. Description of the Related Art
Metal structures such as pipes and tanks, which are buried in soil, are naturally subjected to an electro-chemical corrosion process in their underground environment. The metal structure acts as an electrode and the soil an electrolyte so that an electrolytic cell is formed causing corrosion of the buried structure.
Cathodic protection has commonly been applied to such buried objects in order to counteract and thereby mitigate the electrochemical corrosion reaction. Such cathodic protection systems are monitored in order to determine a level of cathodic protection which is sufficient to adequately mitigate the corrosion of the buried metal object. The level considered sufficient is determined by industry accepted standards, based upon a measurement of the potential difference between the buried object and a standard reference electrode also placed in contact with the soil electrolyte.
One problem with measuring this potential arises from potential differences in the soil resulting from currents through the distributed resistance of the soil. It is necessary to turn off the cathodic protection in order to eliminate the voltage drop arising from the cathodic protection current flowing through the distributed resistance of the soil. However, not only does this interrupt the protection, but it does not eliminate additional voltage drops in the soil which result from stray currents from underground power sources, such as transit systems which cannot be turned off. Additionally, long buried pipelines commonly have a series of several cathodic protection systems spaced along the pipeline. Elimination of the IR drop through the soil from the neighboring cathodic protection systems would require that all cathodic protection systems be turned off during measurement of the pipe to reference electrode voltage.
In order to avoid these problems, coupons are used to monitor the level of cathodic protection on buried metal objects. A coupon is a bare metal electrode having substantially the same metallurgical, and therefore electrochemical, attributes as the buried metal object. The coupon is buried in the soil near the buried metal object so that it is subjected to the same corrosive effects and the same cathodic protection as the buried metal object. Consequently, the coupon is used to simulate the buried metal object and potential difference measurements are taken between the coupon and the reference electrode. This allows the application of cathodic protection to the buried metal object to continue uninterrupted, while only the cathodic protection to the coupon is interrupted for purposes of taking measurements. The coupon reference potential therefore simulates the potential of a coating defect or holiday on the buried metal object.
In order to evaluate such a cathodic protection system there are three electrical parameters which are commonly measured. The first is the electrical current between the coupon and the pipe, the second is the voltage, E.sub.ON, between the pipe and the reference electrode when current is flowing between the coupon and the pipe, and the third is the voltage, E.sub.OFF, between the coupon and the reference electrode at some standard time delay after the coupon-pipe current is interrupted.
These electrical parameters are conventionally measured by an individual technician travelling into the field and connecting a strip chart instrument to the coupon, pipe and reference electrode connections provided at the site of the coupon and reference electrodes. The technician connects the instruments, takes readings and interrupts the electrical connection between the pipe and the coupon to obtain a measurement of E.sub.OFF. The technician must then examine the strip chart and visually determine the appropriate values to record.
One difficulty with such manual measurement is that the values fluctuate with time. The technician must follow the time line of the strip chart for the time delay of the standard test utilized by the particular company responsible for cathodic protection, and estimate the appropriate E.sub.OFF voltage by visual inspection of a meandering line on a strip chart. This manner of measurement is not only inherently inaccurate because of its dependence upon visual inspection and mental interpolation, but also requires training and skill on the part of the technician who is going into the field to make the measurements.
It is therefore an object and feature of the invention to provide a coupon monitor which is capable of making automated measurements of these corrosion coupon parameters.
It is a further object and feature of the invention to allow complex, interrupted potential and current measurements to be easily detected and recorded in the field using a simple digital voltmeter giving a direct reading of the detected potentials and current, so that these can be reliably recorded by persons who do not possess as much skill as currently required.
It is a further object and feature of the invention to provide a coupon monitor which can be attached to a data acquisition system so that a time sequence of parameters can be stored, subsequently downloaded, and utilized by a digital data processing system.